Thymus activity, vitamin D, and respiratory infections in adolescent swimmers

Background: Several studies have identified associations between low vitamin D concentrations and risk of upper respiratory infections (URI). T lymphocytes have a major anti-viral role, are affected by vitamin D metabolism, and may mediate the link between vitamin D and URIs. Competitive swimmers have a relatively high rate of URIs, alongside a high prevalence of low vitamin D concentration. Objectives: To examine the associations linking T cell receptor excision circles (TREC, markers of thymus activity), circulating 25(OH)D concentrations and the effect of vitamin D supplementation, and URI symptoms in young competitive swimmers. Methods: We tested 82 adolescent swimmers for serum 25(OH)D and TREC concentrations and found that 55 had vitamin D insufficiency. Randomized supplementation of either vitamin D3 or placebo was given for 12 winter weeks. URI symptoms were recorded weekly. The associations between TREC copy numbers, vitamin D and URI burden were examined. Results: TREC concentrations decreased with the participants’ age (r = -0.346, P = 0.003), with no significant between-gender difference. TREC concentrations did not materially differ among subjects with normal, insufficient or deficient vitamin D status, and were not affected by vitamin D supplementation. No significant correlations were found between TREC levels or their changes during the study period, and mean URI severity or duration. Conclusions: Thymus activity, represented by higher TREC levels, was not related to vitamin D concentrations or status, and was not affected by vitamin D supplementation in adolescent swimmers. TREC concentrations were not associated with URI severity or duration in this population.